Components of gas turbine engines, such as the blades and vanes of the hot sections within a gas turbine engine, are generally made of a nickel-based or cobalt-based superalloy for high-temperature strength and fatigue resistance. During operation, the components of the gas turbine engine are typically exposed to harsh environments that can damage the turbine components. Environmental damage can occur in various modes, including damages as a result of heat, oxidation, corrosion, hot corrosion, erosion, wear, fatigue or a combination of several degradation modes.
Today's turbine engine is designed and operated in such a way that the environmental conditions and consequently the types of environmental damages in different regions of a turbine component can vary significantly from one another. As a result, a gas turbine component is protected against environmental damages in different locations by different types of protective coatings.
As an example, FIG. 1 shows the various sections of a typical high-pressure turbine blade. The turbine blade has several sections, including an airfoil, a platform and under-platform regions. In service, the airfoil operates at the hottest regions of the turbine blade and is therefore subject to high-temperature oxidation. “Higher temperature” or “high temperature” as used herein and throughout refers to those temperatures generally recognized to cause oxidation to the airfoil region of a turbine blade during service life. Consequently, protection of the base materials of the airfoil regions generally requires an aluminide coating and optionally a thermal barrier coating. In contrast to the airfoil, the regions under the platform of a turbine engine, such as the shank and root, are exposed to relatively lower temperatures but subject to type II hot corrosion attack. Aluminide coating offers an insufficient protection against type II hot corrosion attack. Moreover, the brittleness of aluminide coating at such lower temperature leads to an accelerated stress-corrosion-cracking failure in those highly stressed regions. As a result, a chromide coating with the enhanced corrosion resistance is generally required in the under-platform region. A typical method to make such location-dependent coatings involves a chromide coating on the external surface of under-platform regions by a chromizing process, and then a subsequent separate aluminide coating on the internal and external surface of the airfoil by an aluminizing process.
The chromide coating provides protection against type II hot corrosion attack generally incurred by the under-platform regions. The type II hot corrosion attack tends to be particularly severe in the under-platform regions where sulfate particulates can accumulate on the surface and service temperature is in the range of 1250-1400° F. The chromide coating can be applied by a pack cementation, vapor phase or slurry chromizing process.
The aluminide coating provides an insufficient protection against type II hot corrosion attack, but offers excellent oxidation resistance at higher temperature. Aluminide coatings are generally formed by enriching the surface of component with aluminum. The formation of aluminide coating generally involves the use of a halide activator and aluminum donor material to generate an aluminizing coating gases, gaseous transport of aluminizing coating gases to the surface of the component being coated, reaction of aluminizing coating gases with the surface of component, and deposition and diffusion of aluminum into the surface of the component.
During the aluminizing process, aluminide coating deposition onto the pre-existing chromide coating should be prevented because any aluminum spillover onto chromide coating tends to significantly degrade the chromide coating's protection against type II hot corrosion attack. The effective way is the use of mask material. The mask material can provide an effective environmental barrier between the pre-existing chromide coating and the aluminizing coating gases during the aluminizing process. Moreover, the mask material should not react with the pre-existing chromide coating and therefore deplete the chromium content from chromide coating during the aluminizing process.
The conventional mask material currently used in the diffusion aluminizing industry was designed and developed to mask the nickel- or cobalt-based superalloy substrate. The conventional mask material typically comprises nickel metal powder or nickel alloy powder that can effectively getter aluminizing coating gases during aluminizing process and therefore prevent the deposition of aluminum onto the surface of superalloy substrate. Those masking compositions in the prior arts are well known and are described, for example, in U.S. Pat. Nos. 4,208,453 and 4,845,139 to Baldi. Those masking materials are commercially available, for example, M-1 maskant, M-7 maskant, and M-17 maskant from APV engineered coatings (Arkon, Ohio). The conventional masking materials are generally effective in preventing deposition of aluminide coating on nickel- or cobalt-based superalloy substrate. However, they are not effective in masking the pre-existing chromide coating during a subsequent aluminizing process as demonstrated in comparative example 1 and comparative example 2. It has been observed that conventional masking materials can cause aluminum spillover onto the underlying chromide coating and/or chemical reaction between mask materials and the chromide coating. Consequently, ineffective masking significantly weakens the performance of the chromide coating against type II hot corrosion attack, thereby shortening its service life.
Thus, an effective mask material is required to prevent the deterioration of the chromide coating onto the selected regions during aluminizing process. In view of the current unmet needs, a mask material is desired to provide an effective environmental barrier between the underlying chromide coating and the aluminizing coating gases during an aluminizing process. Moreover, the mask material should not react with the underlying chromide coating and should not significantly alter or affect the chemistry and microstructure of the underlying chromide coating.